Tuesday, September 7, 2010

Determining Solar Collector Flow Rate

This blog entry deals with picking the best water flow rate for the solar collectors in a solar water or spacing heating system.

There are a lot of recommendations around on what the flow rate through water heating solar collectors should be.

These are some of the recommendations floating around out there:
- Heliodyne recommends a range of 0.025 gpm/sf to 0.075 gpm/sf
- SunEarth recommends 0.025 gpm/sf for their collectors
- Solar discussion groups often talk about 0.02 to 0.03 gpm/sf a good number.
So, what is the "best" number, and what are the tradeoffs?

In a nutshell, if you set the flow rate too low, then the collector will run  hotter than it should, and this will result in larger heat losses and a less efficient collector.  On the other hand, setting the flow rate too high results in a larger pump and larger pipes than are really needed, and this increases both the initial and operating cost of the system.

The table below shows the reduction in collector efficiency and heat output as the flow rate is decreased from a high flow level:

All the details on how these flow rates are arrived at here...

Details on sizing a pump to deliver a given system flow rate ...

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